High density polyethylene (HDPE) is one of the most versatile of the thermoplastic resins and offers a broad spectrum of uses. The properties of finished goods made from HDPE are strongly influenced by the polymerization and processing conditions and the properties of the polymer and the resin.
Physical and mechanical properties of Ziegler HDPE can be summarized as follows: low to medium stiffness and hardness; medium to extremely high toughness; no reduction in toughness to -40.degree. C.; unrestricted usage in contact with food; no restriction for disposal, burning, or recycling; high resistance against solvents and chemicals at ambient temperature; and easy and safe to process.
Molecular weights of polyethylenes are customarily evaluated by measuring their melt indexes in accordance with ASTM D-1238. Three melt indexes are cited below, those measured with the weights of 21.6 kg (I.sub.21 or high-load melt index, HLMI), 5.0 kg (I.sub.5) and 2.16 kg (I.sub.2 or melt index, MI). All these numbers vary inversely with resins' molecular weight.
Strength, as well as impact, stress and puncture resistance, together with toughness, are attributed to high molecular weight resins.
However, as the molecular weight of the resin increases, the processabiliity of the resin usually does decrease. By providing a blend of polymers, the properties characteristic of high molecular weight resins can be retained and processability, particularly extrudability, can be improved.
Various approaches have been examined for production of such blends. Physical blending suffers the disadvantage brought on by the requirement of complete homogenization and attendant high cost. Direct synthesis with one catalyst, although theoretically possible and most desirable, is difficult to achieve. The third strategy is a multi-stage polymerization which involves different staging of variables, usually in multi reactor set ups, sometimes referred to as tandem, which provide the possibility of diversity in molecular weight. The products exhibit good ESCR and stiffness.
However, the tandem produced products have not received universal domestic market acceptance because the tandem produced products exhibit different swell and melt fracture characteristics from the Phillips type resins, requiring downstream equipment modification, with high attendant costs.
In accordance with the invention, it is an object to provide bimodal film with the desired physical properties of the tandem products but with improved bubble stability and film MD and TD tear properties.